Today, it would be much easier to answer the question, because monarchies have standarised forms of address and etiquette. So, only a sovereign, his wife or mother receives "Majesty." A Queen Dowager or Queen Mother would therefore be "Your Majesty." A princess dowager would be "Your Highness" or "Your Royal Highness," like all other members of the royal family.

In the sixteenth century, "majesty" was originally associated far more with European/continental monarchies, like Spain, France or the Hapsburg Empire. There, like kb says, the Queen Mothers, like Catherine de Medici, were referred to as "Your Majesty." In England, Henry VIII was the first monarch who introduced "majesty" to be used as it was in Europe. Before that, it would either have been "highness" or "grace" for both women.